CHICAGO (Reuters) - U.S. physicists have made a clock so
accurate it will neither gain nor lose even a second in more
than 200 million years, a finding sure to please even the most
punctually minded.

The clock, described in the Friday issue of the journal
Science, outperforms the official atomic clock used by the U.S.
Commerce Department's National Institute of Standards and
Technology, which promises to keep accurate time down to the
second for 80 million years.

The new atomic clock is vying for the title of world's most
accurate with another experimental clock developed in the same
lab at the Joint Institute for Laboratory Astrophysics, a
collaboration between NIST and the University of Colorado in
Boulder.

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"These clocks are improving so rapidly that it is
impossible to tell which one will be the best," said Tom
0'Brian, head of the Time and Frequency Division at NIST.

Such highly precise clocks are critical for deep space
navigation, where even a slight error can make or break a space
mission.

The secret to making an extremely accurate clock is
speeding up how fast it ticks. "If you make a mistake, you can
know about that mistake very fast," said Jun Ye, who developed
the atomic clock at JILA.

Ye's clock has 430 trillion "ticks" per second.

Its pendulum uses thousands of strontium atoms suspended in
grids of laser light. This allows the researchers to trap the
atoms and measure the movement of energy inside.

"Essentially, we are probing the energy structure of the
atom. We are probing how electrons make transitions between a
set of energy levels," Ye said in a telephone interview.

"This is the time scale that was made by the universe. It
is very stable."

To test his clock's accuracy, Ye and colleagues compared it
with another optical atomic clock -- this one measuring calcium
atoms. This calcium clock is highly stable only over short
periods of time, so the researchers had to make fast
measurements for their comparisons.

Next Ye wants to take on a clock that measures a single
ion, or charged particle, of mercury. This clock, also
developed at JILA, was accurate to about 1 second in 400
million years in 2006. Because Ye's clock measures thousands of
atoms at once, it produces stronger signals, something Ye
thinks may give him an edge.

"These clocks are among the best in the world," John Lowe,
leader of the atomic standards group at NIST, said in a
telephone interview. "Longer-term experiments will prove which
of these clocks may end up becoming the next standard of
international agreement."

Ye said pushing for ever more accurate clocks will allow
physicists to test some of the basic questions about the nature
of the universe.

It also can be used to synchronize telecommunications
networks and might some day lead to things like hands-free
driving in satellite-guided cars.

"If we can navigate a vehicle on Mars and ask it to settle
down on a particular runway, I'm sure we can navigate all the
cars on Earth with satellites," Ye said.